Transient axonal projections develop during the normal ontogenesis of pathways in the central nervous system. We have identified a transient projection from lamina V neurons in the primary sensorimotor cortex to the cerebellar cortex and nuclei in neonatal kittens. Neurons in face, forelimb, and hindlimb areas of the primary somatosensory cortex (S-I) project to somatotopically homologous areas of the cerebellar cortex. Many cerebrocerebellar projections are transient collaterals of corticobulbar and corticospinal axons which persist. After reaching their maximum density in the cerebellum in the second postnatal week, all cerebrocerebellar projections are normally eliminated. The regressions of some transient pathways has been shown to be influenced by impulse activity, and there are findings that pathways which are functionally silent are eliminated. We propose two sets of experiments to determine if transient cerebrocerebellar projections are eliminated as the result of: (1) the cell bodies not being respoonsive to orthodromic activation or spontaneously active, (2) cerebrocerebellar axons not propagating action potentials, (3) cerebrocerebellar projections not forming functional synaptic contacts with cerebellar neurons. In the first set of experiments, we will use extracellular unitary recordings to determine the physiological characteristics of cerebrocerebellar projection neurons in S-I. In the second set of experiments, we will determine electrophysiologically if cerebrocerebellar axons propagate action potentials and if stimulation of the cerebral cortex functionally activates cerebellar neurons. In other experiments, we will use EM autoradiographic techniques to determine if cerebrocerebellar axons, orthogradely labeled from tritiated amino acid injection sites in the cerebrum, form synaptic contact with cerebellar neurons. The elimination of transient cerebrocerebellar projections is not predetermined and many will persist after other cerebellar afferents are directly damaged. In a third set of experiments, we will determine if cerebrocerebellar axons persist after lesions which indirectly deafferent the paramedian lobule of face somatosensory input. We anticipate that only projections from the face area of S-I will persist after trigeminal primary afferents are cut, indicating that the stabilization of these projections provides an alternate circuit for functionally relevant information to reach the cerebellum.